Let’s examine the components of a cell in more detail.
Membrane A Polymer Electrolyte Membrane (PEM)is a solid-state barrier between the electrodes. PEM cells operate most efficiently at 2 volts. But they can utilize a variable current input, making them attractive to pair with renewable power sources that can fluctuate by time of day or weather conditions. Their efficiency leads to reduced operational costs. The membrane's low gas crossover rate results in very high product gas purity, critical for storage safety.
Cells are stacked, i.e., connected electrically in a series to increase production. Stacked PEM electrolyzers can range in size from small tabletop models to those of 2 MW capacity that fit into a standard shipping container. Multiple units can be hooked up to a common power source to create 10, even 20 or more MW H2 generation plants.
Alkaline electrolyzers utilize a solution of sodium or potassium hydroxide as the electrolyte to separate the electrodes. While not as efficient as PEM technology, they have the advantage of operating at lower temperatures. Extra filtration is required to maintain gas purity. Alkaline electrolyzer cells operate most efficiently at1.5volts.
Solid oxide ceramic electrolyzers work at much higher temperatures, about 700°-800°C, compared to PEMs. They operate at the lowest voltages, but obviously require a greater investment in heating equipment.
Of the three technologies discussed here, most manufacturers are pursuing PEM.
Cathode Platinum, palladium, rhodium, ruthenium, iridium, and osmium are the materials of choice for electrolyzer catalysts. These elements best resist the acidic electrolyte medium. Their great disadvantage is these metals are rare and pricey. But recent experiments have shown success with iron doped with carbon and nitrogen. In the end, deployment to the moon and Mars will probably be a Trade-off. Which is more expensive, the material cost of the platinum catalyst, or the transportation cost the heavier iron-based alternative?
Power Source Most power sources for green hydrogen are intermittent and variable. The wind may blow at hurricane force, be a gentle breeze or be calm--all on the same day. Solar intensity varies by time of day, cloud cover and season. Manufacturers follow one of two strategies to maintain constant voltage to their stacks. Current can be regulated outside the stack, but at a loss of efficiency typical of transformers and voltage regulators. Or modulation occurs within the stack by precisely matching the number of cells to the amount of current instantaneously available.
Two potential power sources could be deployed for use on the moon or Mars. A dedicated solid state nuclear reactor would produce a steady voltage and amperage. It could be packaged as a unit with the electrolyzer. The trade-off would be high mass at relatively compact volume. A solar array would weigh less but require greater current modulation. One of comparable capacity to a nuclear fission battery would occupy much more room on board a spacecraft.
The world-wide clean hydrogen generation industry is expected to exceed a trillion dollars market value by the end of this decade. Competition within the water electrolysis sector is fierce. Large US players include companies such as Cummins, Teledyne Technologies, and Plug Power. Europe and Asia are also home to significant manufacturers. Numerous startups are pursuing materials and components with greater durability, electrical efficiency, and lower cost.
Will water electrolyzers go to the moon with Artemis? According to NASA, Shackleton Crater is the prime target for Artemis Base Camp due to its wide variety of lunar geography and water ice. This ice will be a resource for drinking, growing crops and electrolyzing oxygen to breathe and hydrogen (and oxygen) for rocket fuel.
Prior to the first human landing, NASA plans to send the Volatiles Investigating Polar Exploration Rover (VIPER)to the lunar South Pole. The mobile VIPER will discover and map the distribution and concentration of ice that could eventually be harvested to support forays farther into the solar system.
NASA anticipates the establishment of a permanent base by the end of the 2020s. Its power source will be a 10 kW nuclear fission reactor. While not explicitly stated, an experimental PEM electrolyzer will likely be set up as a proof of concept for future Mars missions.
Here on Earth, the US Department of Energy's goal for green hydrogen production is 10 million tons per year by2030. This is equal to today's use of fossil fuel-based gray hydrogen, consumed in fertilizer production and heavy industrial processes. Annual production will increase to 50 million tons by 2050, allowing direct replacement of fossil fuels. Numerous demonstrations are operating right now. Many industrial-scale projects will come on-line within the next two years.
While I don't get into the technical details as I have in this article, PEM technology is prominently featured in my EPSILPN SciFi thriller series books. Look for it in my upcoming novel Blood Moon, available on Monday, November 28th.
For Further Readinghttps://www.energy.gov/eere/fuelcells/hydrogen-production-electrolysishttps://www.allens.com.au/insights-news/insights/2021/10/Water-access-for-hydrogen-projects/https://www.hydrogenfuelnews.com/hydrogen-fuel-cells-ub/8553489/https://www.hydrogenfuelnews.com/electrolyzer-design-variable/8555454/?awt_a=1jpsU&awt_l=5TI0C&awt_m=hmZVapKEsO5DlsUhttps://en.wikipedia.org/wiki/Polymer_electrolyte_membrane_electrolysishttps://en.wikipedia.org/wiki/Artemis_programhttps://www.nasa.gov/specials/artemis/https://blogs.nasa.gov/artemis/2020/10/28/lunar-living-nasas-artemis-base-camp-concept/
